1. Field of the Invention
The present invention relates to a network apparatus that functions as an interface on the side of a WAN (wide area network) and links a LAN (local area network) and the WAN by a plurality of lines through a plurality of ports.
2. Description of the Related Art
A technology known as SONET/SDH (Synchronous optical network/synchronous digital hierarchy) has been conventionally used to create WAN (wide area network) by connecting LAN (local area networks) formed from user terminals. SONET/SDH enables high-speed data transfers over optical communication lines, accounting for reliable WANs due to redundancy of transmission channels.
In contrast to the high reliability of SONET/SDH, Ethernet (registered trademark) used in LANs has a relatively low reliability. Thus, when Ethernet (registered trademark) and SONET/SDH are connected, communication failure could occur due to malfunctioning Ethernet (registered trademark).
To address the problem mentioned above, as described in International Publication No. WO/2005/079015, a linking device on the Ethernet (registered trademark) network side in the form of an Ethernet (registered trademark) device and a linking device on the SONET/SDH network side in the form of an SDH device are linked by an active line and a standby line redundantly, wherein when communication failure is detected in the active line between the Ethernet (registered trademark) device and the SDH device, the SDH device resumes communication by switching to the standby line.
A technology known as link aggregation is generally used for multilinking the Ethernet (registered trademark) device and the SDH device. Link aggregation allows multiple physical lines to be treated as a single logical line, thus increasing the effective bandwidth of communication channels. In link aggregation, packet data should be transmitted from the Ethernet (registered trademark) device to the SDH device without specifying any line. Thus, merely switching the lines from active to standby on the SDH device side can result in loss of packet data transmitted from the Ethernet (registered trademark) device to the SDH device.
Thus, in the conventional technology represented by the technology disclosed in International Publication No. WO/2005/079015, packet loss occurs due to switching of the lines from active to standby on the SDH device side.